throque

Simple async function decorator which limits how many invocations of said function can be active at once.

Pronounced: throck

Etymology: Throttle + Queue = throque

Installation and usage

yarn add throque or npm install --save throque

const throque = require('throque');
const writeFileThroqued = throque(fs.writeFile, 200, false);

History

throque was born from this pesky Node.js error: Error: EMFILE: too many open files. This error gets thrown when Nodes' file-system module has too many open files at once (obviously.) throque alleviates this problem by creating a decorated version of your function which Queues excessive calls until the appropriate conditions are met.

Example

This snippet throws the error mentioned above:

for (let i = 0; i < 10000; i += 1) {
  fs.readFile(filePath, { encoding: 'utf8' }, err => {
    if (err) throw err;
  });
}

throque to the rescue:

const throque = require('throque');
const readFileThroqued = throque(fs.readFile, 200, false);

for (let i = 0; i < 10000; i += 1) {
  readFileThroqued(filePath, { encoding: 'utf8' }) // We can still pass the same arguments to fs.readFile (but not the callback)
    .then(console.log)   // That's a lot of logs
    .catch(console.log); // No error! :)
}

Notice how we are no longer using callbacks when using a throqued function. This is because throque will promisify your functiontoThrottle if you set the returnsPromise argument to false. There are some important things to note when setting this flag to false however (see API below.)

API

throque currently exports a single function which is relatively simple to use.

throttleAndQueue aka throque

throque(functionToThrottle, maxConcurrentCalls, returnsPromise);

Parameters

functionToThrottle: function

maxConcurrentCalls: number (default: 100)

• Maximum amount of invocations of the throttled function that can be active at any given time.

returnsPromise: boolean (default: true)

• Indicates whether or not your functionToThrottle returns a promise (true) or calls a callback when finished (false).
• If this flag is set to false, there are some important things to note:
  • The returned throttled function will be a promisified version of functionToThrottle.
  • functionToThrottle must follow the Node.js error-first callback pattern, or the internal promisification will fail. Even though Node native library methods should all work perfectly fine this way, it is reccomended to set this value to true (which means your functiontoThrottle returns a promise.)
  • If your functionToThrottle only passes a single argument to its' callback, your promisified function will resolve with this value, otherwise it will resolve with an array of values in the same order that would of been passed into the callback. You can use destructuring assignment to mimic the callback signature.

Plans for the future

Tests

Currently throque is not tested, but tests will be added soon, and future additions to the library will be developed test-first.

Extensive error handling

Although tracking errors at the time of writing should be extremely simple (the library is very small), more robust and descriptive error handling will be added.

Features

• Ability to group multiple functionsToThrottle into one throttle-queue.

• Tracking calls made and ability to generate reports.